1. Field of the Invention
The present invention relates to a single crystal pulling apparatus for growing and pulling up a single crystal by the Czochralski method (hereafter referred to as the “CZ method”)
2. Description of the Related Art
The CZ method is widely used for growing a silicon single crystal. This method is such that a seed crystal is brought into contact with a surface of silicon liquid melt which is accommodated in a crucible, the crucible is rotated about a pull-up axis, and this seed crystal is pulled upwards and rotated in the opposite direction, whereby a single crystal is formed at a lower end of the seed crystal.
As shown in FIG. 6, in a pull-up method using a conventional CZ method, firstly a silica glass crucible 51 is filled with material polysilicon which is heated by a heater 52 so as to be silicon liquid melt M. After an appropriate time, a seed crystal P attached to a pulling wire 50 is brought into contact with the silicon liquid melt M, so that a silicon single crystal C is pulled up.
Generally, after a temperature of the silicon liquid melt M is stabilized in advance of starting the pulling, a necking process is carried out in which the seed crystal P is brought into contact with the silicon liquid melt M to melt a tip part of the seed crystal P, as shown in FIG. 7. By the necking process we mean an indispensable process for removing a dislocation produced in the silicon single crystal by a thermal shock generated due to contact between the seed crystal P and the silicon liquid melt M. A neck part P1 is formed by way of this necking process. Further, this neck part P1 generally requires a diameter of 3-4 mm and a length of 100 mm or more.
Further, as for processes after starting the pulling, a crowning process of extending a crystal to the extent of the straight body part diameter after completing the necking process, a straight body forming process of growing the single crystal used for a product, and a tail process of gradually reducing a single crystal diameter after the straight body forming process are performed.
Incidentally, the silicon single crystal obtained by way of such processes contains oxygen which is eluted from the silica glass crucible into the silicon liquid melt. The oxygen in this single crystal appears as an inner minute defect (bulk micro defect: BMD) which includes an oxygen deposit (core) in a wafer sliced from the single crystal. Since BMD can be used for an IG method (method of gettering impurities from inside) where a contaminating heavy metal included in the wafer is captured with distortion stress, it is preferable that a BMD density in the wafer is higher.
Further, Japanese Patent Publication (KOKAI) No. 2003-249501 (patent document 1) discloses that the wafer which is subjected to pre-annealing (heat treatment) to increase the BMD density can be inhibited from generating slip dislocations near the outermost perimeter and a holder of the wafer, even if it is subjected to a subsequent high temperature heat treatment. In other words, it is desirable to increase the BMD density in the wafer also from a viewpoint of dislocation inhibition effects.
As described above, it is desirable that the BMD density in the wafer is in a high state in terms of gettering the impurities and inhibiting the dislocations. To this end, however, it is necessary to grow a single crystal with a high oxygen concentration, preferably a single crystal having an oxygen concentration of 1.1-1.5×1018 atoms/cm3 over the whole length.
However, since the single crystal to be grown has been increased in diameter in recent years, convection of the silicon liquid melt in the crucible becomes irregular, the concentration of the oxygen or impurities carried by the convection and a liquid melt temperature at a crystal growth side becomes unstable, thus it is not possible to stably manufacture the single crystal having the high oxygen concentration over the whole length.
To cope with such a problem, a method is used in which a magnetic field is applied to the silicon liquid melt to inhibit the convection of the silicon liquid melt in the case of pulling the single crystal with a large diameter. However, it is difficult to obtain the single crystal with the high oxygen concentration (for example, 1.1-1.5×1018 atoms/cm3) over the whole length, merely by applying the magnetic field.